Overlay Dataset

ABSTRACT

Overlay datasets provide an efficient, flexible and scalable mechanism to represent the logical replication of one or more prior defined datasets. Only changes made to an entity in an overlay dataset&#39;s underlying dataset are replicated into the overlay dataset (such changes do not affect the underlying dataset). Read operations directed to the overlay dataset will find entities in the overlay dataset if they exist and in the underlying dataset(s) if no overlay-specific entity exists. Accordingly, overlay datasets provide an efficient mechanism for making changes to an existing dataset without suffering the high processing time and storage overhead associated with prior art copying and versioning techniques. Overlay datasets also provide a natural mechanism to keep two or more datasets in synchronization because changes to a base or underlying dataset&#39;s entities are “visible” in its associated overlay dataset (unless the entity has been modified in the overlay dataset).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority under 35 U.S.C.§120 to U.S. application Ser. No. 11/538,377, filed on Oct. 3, 2006, andentitled “OVERLAY DATASET”, which claims priority to U.S. provisionalpatent application 60/745,870, entitled “Overlay Dataset,” filed 28 Apr.2006, both of which are hereby incorporated by reference. U.S.application Ser. No. 11/538,377, filed on Oct. 3, 2006, and entitled“OVERLAY DATASET”, is also related to U.S. patent application Ser. No.11/204,189, entitled “Resource Reconciliation,” filed on Aug. 15, 2005,which is hereby incorporated by reference.

BACKGROUND

In the vernacular of database technology, a “dataset” is a collection ofrelated data or information and their relationships, that are organizedand treated as a unit. One illustrative dataset is the data collected bya given sensor or collection of sensors. Another illustrative dataset isthe collection of database entities (e.g., objects in an object-orienteddatabase) related to a given task. In the context of configurationmanagement, a dataset may be described as a collection of data, andtheir relationships, that together represent information from a givensource. In this environment, a dataset could be the collection ofconfiguration items, and their relationships, obtained from a givennetwork discovery source.

One of ordinary skill in the art of database and/or configurationmanagement will recognize that, for various reasons, it is sometimesuseful to have a second dataset based on a given (first) dataset,wherein one or more characteristics of the second dataset are changedwith respect to the first dataset without affecting the first dataset.In the prior art, second datasets are provided by either copying orversioning an original dataset. Copying is expensive both in terms ofthe time it takes to copy all instances of a dataset and in terms of thestorage required to retain the duplicate information (especially forlarge datasets). Copying also has the drawback of loosing the connectionbetween the instances in the two datasets so that the two environments(i.e., the first and second datasets) can start diverging almostimmediately—especially when the operational environment is dynamic. Itwill also be recognized that copying suffers from a scalability problem.Versioning creates copies of data instances (e.g., entities or objects)as they are changed, establishing a version for each new copy. Differentdatasets can then be created post hoc by gathering together thoseconfiguration items with specific version tags or based on a time. Asignificant drawback to versioning is its lack of flexibility—it isdifficult to have multiple parallel copies of a common dataset, eachwith its own (typically small) perturbations.

Thus, it would be beneficial to provide a mechanism whereby a second oroverlay dataset could be specified that is a “duplicate” of a firstdataset except for one or more specified changes that avoids ormitigates the noted drawbacks to prior art duplication techniques (e.g.,copying and versioning).

SUMMARY

In one embodiment the invention provides a method to use an overlaydataset. The method includes receiving a request for a database entityassociated with the overlay dataset (wherein the overlay dataset mayinclude one or more base, underlying or member datasets), returning theentity from the overlay dataset if it exists as a unique entry in theoverlay dataset, otherwise returning the entity from one of the one ormore base datasets. Methods in accordance with the invention may bestored in any media that is readable and executable by a computersystem.

In another embodiment, the invention provides a data structure stored ina computer readable medium for use in a database. The data structureincludes a first value uniquely identifying the data structure, a secondvalue identifying the data structure as being associated with an overlaydataset and a third value identifying one or more additional datasets,wherein each of the one or more additional datasets are associated asmembers of the overlay dataset.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C illustrate, in block diagram form, access operations throughan overlay dataset in accordance with one embodiment of the invention.

FIG. 2 shows, in flowchart form, an overlay dataset access technique inaccordance with one embodiment of the invention.

FIG. 3 shows, in block diagram form, an overlay dataset in accordancewith the invention defined in terms of another overlay dataset.

FIG. 4 shows, in flowchart form, an overlay dataset access technique inaccordance with another embodiment of the invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention as claimed and is provided in thecontext of the particular examples discussed below, variations of whichwill be readily apparent to those skilled in the art. Accordingly, theclaims appended hereto are not intended to be limited by the disclosedembodiments, but are to be accorded their widest scope consistent withthe principles and features disclosed herein.

An overlay dataset in accordance with the invention provides anefficient, flexible and scalable mechanism to represent the logicalreplication (with changes) of one or more prior defined datasets. In oneembodiment, an overlay dataset has all the characteristics of any otherdataset. On creation, however, an overlay dataset has no instances,elements or records—just a reference to the dataset(s) it is an overlayfor. When the overlay dataset is accessed, if the target entity has notbeen modified from an underlying, base, source or member dataset, ornewly added to the overlay dataset, the entity from the underlyingdataset is returned. Thus, access of an unmodified entity in an overlaydataset will “read through” the overlay dataset to the underlyingdataset to retrieve the target entity. When an entity in one of theoverlay dataset's underlying datasets is modified through the overlaydataset (including any associated relationships), that entity is copiedor instantiated in the overlay dataset. When such an entity is targetedfor access through the overlay dataset, the overlay dataset's copy ofthe entity is returned. Thus, modified entities “mask out” entries inthe underlying dataset(s). It will be recognized that an entity orobject may be modified by having one or more of its associated values,attributes or relationships modified or by being designated as deleted.Entities from an underlying dataset designated as deleted in the overlaydataset may be instantiated in the overlay dataset and flagged or markedas deleted. While such entities may be identified during overlay datasetaccess operations, they are not generally returned (even though they maycontinue to exist in the underlying dataset). Finally, entities added tothe overlay dataset (and do not, therefore, exist in any of its base,source, underlying or member datasets) are accessible through theoverlay dataset only. As used herein, the term “entity” indicates adatabase entry in its most general form. In an object-oriented database,for example, an entity could be an object while in a relationaldatabase, an entity could be a record.

Referring to FIGS. 1A-1C, the above-described behavior may beillustrated by considering database 100 that includes overlay dataset105 which itself includes base, underlying, source or member dataset 110having entities 115 and 120. As shown in FIG. 1A, when initially createdoverlay dataset 105 includes no entitles unique to itself so that, forexample, access 125 through overlay dataset 105 for entity 120 returnsentity 120 as it exists in base, member or source dataset 110. Referringto FIG. 1B, at some later time entity 120 may be modified 130 throughoverlay dataset 105, resulting in entity 120′ being instantiated orcreated within overlay dataset 105. Referring now to FIG. 1C, followingthe modification of entity 120 through overlay dataset 105 to createentity 120′, any subsequent access 135 for entity 120 through overlaydataset 105 will return entity 120′ while access to non-modifiedentities through overlay dataset 105 continue to return entities as theyexist in their base, underlying, source or member dataset (e.g., access140 to entity 115).

One of ordinary skill in the art will recognize that in practice adataset may include thousands or millions of separate entities orobjects and that each such object may participate in zero or morerelationships with other entities. In addition, overlay datasets may bebased on any number of underlying datasets. Accordingly, FIG. 1represents a very simplified or schematic view of an actual embodiment.

Referring to FIG. 2, dataset access technique 200 in accordance with oneembodiment of the invention begins when a request for a specified entityfrom a designated dataset is received (block 205). If the specifiedentity is found in the designated dataset (the “Yes” prong of block210), a check is made to determine if the entity has been marked asdeleted (block 215). If it has (the “Yes” prong of block 215), an errormessage is returned indicating the specified entity is not availablethrough the designated dataset (block 220). If the found entity is notmarked as deleted (the “No” prong of block 215), the specified entity isreturned (block 225). If the specified entity is not found in thedesignated dataset (the “No” prong of block 210), the designated datasetis checked to determine if it is an overlay dataset (block 230). In oneembodiment, for example, a dataset is a data structure that includesmetadata indicating whether it is an overlay dataset (e.g., an “overlay”flag attribute or value). If the designated dataset is not an overlaydataset (the “No” prong of block 230), an error message is returnedindicating the specified entity could not be found (block 220). If thedesignated dataset is an overlay dataset (the “Yes” prong of block 230),the overlay dataset's base or source dataset is set to be the designateddataset (block 235), where after processing continues at block 210. Itwill be appreciated that operations in accordance with block 235 may beinvoked for each member dataset comprising an overlay dataset.

On this point, it is further noted that an overlay dataset in accordancewith the invention is not limited to being comprised of non-overlay(prior art) datasets. Referring to FIG. 3, for example, overlay dataset300 may be defined in terms of one or more previously defined overlaydatasets such as overlay dataset 305, itself defined in terms ofnon-overlay, or prior art, datasets 310 and 315, as well as zero or morenon-overlay datasets such as dataset 320.

Referring to FIG. 4, dataset access technique 400 in accordance withanother embodiment of the invention begins as before when a request fora specified entity from a designated dataset is received (block 205). Asearch is then performed for the specified entity in the designateddataset and any overlay datasets that are members of the designateddataset (block 405). The results are then placed in overlay order (block410). As used herein, “overlay order” refers to a sequence whereinentities instantiated in an overlay dataset come before their namesakeentities in the overlay dataset's underlying or base dataset. Thisordering may be recursive if an underlying or source dataset is itselfan overlay dataset. This ordering may be user-specified or automatic asdescribed above. If the specified entity is found in the result setgenerated in accordance with block 410 (the “Yes” prong of block 415), afurther check is made to determine if the specified entity has beenmarked as deleted (block 420). If so marked (the “Yes” prong of block420), an error message is returned indicating the specified entity isnot available through the designated dataset (block 425). If the entityis not marked as deleted (the “No” prong of block 420), the first-mostentity in the result list in accordance with block 410 is returned(block 430). It will be recognized that if the designated dataset is anoverlay dataset and the specified entity is a modified form of an entityfrom an underlying or base dataset, there will be more than one“specified” entity in the result set. If the entity is not found in theresult list generated in accordance with block 410 (the “No” prong ofblock 415), an error message is returned indicating the entity could notbe found (block 425).

In summary, from an access perspective, an overlay dataset is simplyanother dataset and can be accessed and updated as such. From a systemperspective, an overlay dataset is a facade over one or more specified,underlying or source datasets. Changes made to the overlay dataset occurwithin the overlay dataset only and do not affect the underlyingdataset(s). Read operations directed to the overlay dataset will findentities in the overlay dataset if they exist and in the underlyingdataset(s) if no overlay-specific entity exists. Accordingly, overlaydatasets in accordance with the invention provide an efficient mechanismfor making changes to a an existing dataset without suffering the highprocessing time and storage overhead associated with prior art copyingand versioning techniques. In addition, entities in an underlying,source or base dataset that are not expressly modified in the overlaydataset are inherently synchronized in the overlay dataset. That is,changes to these entities in the underlying datasets are intrinsicallyvisible when using the overlay dataset (unless the entity has beenexplicitly modified in the overlay dataset).

By way of example, overlay datasets have been implemented in the BMCAtrium™ CMDB product—a configuration management database product. (BMCATRIUM is a trademark of BMC Software, Inc. of Houston, Tex.) It will berecognized by one of ordinary skill that a configuration managementdatabase is a database that contains information about the components inan organization's information system and the relationships between thosecomponents. Such components, within the context of a configurationmanagement database, are generally referred to as configuration items.Thus, configuration items are software structures that representinformation technology components. Illustrative configuration itemsrepresent: software applications, patches and modules; complete computersystems; components within a computer system such as storage units andnetwork switches; people; departments; computer networks; and therelationships between different configuration items.

The BMC Atrium CMDB product utilizes an object-oriented model on arelational database whose elements are defined in terms of a series ofobjects organized in accordance with a common data model. As shown inTable 1, one embodiment of a dataset object in accordance with theinvention includes two attributes that implement the overlay concept.The DataSetType attribute simply identifies a dataset as being anoverlay dataset or a non-overlay dataset. The SourceDatasetId identifiesthe dataset which is the overlay dataset's underlying, base, source ormember dataset. In another embodiment, the SourceDatasetId attribute maybe a semicolon delimited list of unique dataset identifiers—therebypermitting more than one dataset to be a base, underlying or sourcedataset. In addition, each object class such as a collection (e.g., anorganization), a logical entity (e.g., a business service), a systemcomponent (e.g., a storage disk) or system (e.g., an application suite)has a dataset identifier attribute. When a configuration item isinstantiated, its dataset identifier attribute is assigned a value thatuniquely identifies the dataset to which it belongs. This attributeprovides the “glue” which associates individual configuration items witha dataset.

TABLE 1 Example Dataset Object ATTRIBUTE TYPE COMMENT AccessibilityInteger A first value (e.g., ″0″) indicates the dataset is writable byany client - that is, configuration items may be added to the dataset. Asecond value (e.g., ″1″) indicates the dataset is read-only for allclients. A third value (e.g., ″2″) could be ′client-dependent′ such thatonly those clients explicitly identified here (or in another attribute,not shown) are permitted to have write access. CoreDatasetId CharacterDataset's unique identifier. DatasetType Integer A first value (e.g.,″1″) indicates the dataset is on overlay dataset. A second value (e.g.,″0″) indicates the dataset is a non-overlay dataset. Name Character Nameof dataset. SourceDatasetId Character Identifier for the underlying,base or source dataset.

In the illustrative embodiments described above, if any attribute of anentity was modified through an overlay dataset, the entire entity(including its relationships) is replicated into the overlay datasetwith the designated changes being made. In other embodiments, however,overlay dataset granularity may be at the attribute or “aggregateentity” level. At the attribute level, only those specific changes to anentity's attributes (including relationships) are replicated into theoverlay dataset with all non-modified attributes being retained in thebase or underlying dataset's entity. At the aggregate entity level, ifany attribute to a specified collection of entities is modified (e.g., acomputer system comprising a number of different components, each ofwhich may be associated with an entity/configuration item), the entirecollection of entities is replicated into the overlay dataset.

Thus, various changes in the structure as well as in the details of theillustrated operational methods are possible without departing from thescope of the following claims. For example, overlay datasets may beimplemented in program code and incorporated in a database managementsystem or configuration management database. Further, acts in accordancewith FIGS. 2 and 4 may be performed by a programmable control deviceexecuting said program code. A programmable control device may be asingle computer processor, a special purpose processor (e.g., a digitalsignal processor, “DSP”), a plurality of processors coupled by acommunications link or a custom designed state machine. Custom designedstate machines may be embodied in a hardware device such as anintegrated circuit including, but not limited to, application specificintegrated circuits (“ASICs”) or field programmable gate array(“FPGAs”). Storage devices suitable for tangibly embodying programinstructions include, but are not limited to: magnetic disks (fixed,floppy, and removable) and tape; optical media such as CD-ROMs anddigital video disks (“DVDs”); and semiconductor memory devices such asElectrically Programmable Read-Only Memory (“EPROM”), ElectricallyErasable Programmable Read-Only Memory (“EEPROM”), Programmable GateArrays and flash devices.

1. A computer-implemented method comprising: creating a first overlaydataset that references an underlying base dataset included in aplurality of base datasets, the base dataset including a plurality ofentities, the creating of the first overlay dataset including setting adataset type attribute included in a first dataset object associatedwith the first overlay dataset to an overlay dataset value, the overlaydataset value identifying a dataset as an overlay dataset; creating asecond overlay dataset that references the first overlay dataset, thefirst overlay dataset, the second overlay dataset, and the plurality ofbase datasets being collections of data included in a database, thecreating of the second overlay dataset including setting a dataset typeattribute included in a second dataset object associated with the secondoverlay dataset to an overlay dataset value, the overlay dataset valueidentifying a dataset as an overlay dataset; receiving a request foraccess to a first entity included in the underlying base dataset, therequest designating a first dataset; accessing the designated firstdataset to determine whether the first entity is included in thedesignated first dataset; responsive to determining that the firstentity is not included in the designated first dataset, determining thatthe designated first dataset is the second overlay dataset based in parton determining that the dataset type attribute included in the seconddataset object is set to the overlay dataset value; accessing the firstoverlay dataset to determine whether the first entity is included in thefirst overlay dataset; responsive to the accessing of the first overlaydataset: determining that the first entity is not included in the firstoverlay dataset; and determining that the dataset type attributeincluded in the first dataset object is set to the overlay datasetvalue; retrieving the first entity from the underlying base dataset, theretrieving comprising reading through the second overlay dataset and thefirst overlay dataset to the underlying base dataset; and returning thefirst entity retrieved from the underlying base dataset in response tothe request for access to the first entity that designated the firstdataset.
 2. The computer-implemented method of claim 1, furthercomprising: receiving a request for access to a second entity, therequest designating the second overlay dataset; determining that thesecond entity is included in the second overlay dataset; retrieving thesecond entity from the second overlay dataset; and returning the secondentity retrieved from the second overlay dataset in response to therequest for access to the second entity that designated the secondoverlay dataset.
 3. The computer-implemented method of claim 2, furthercomprising: determining that a copy of the second entity is included inthe first overlay dataset; retrieving the copy of the second entity fromthe first overlay dataset; generating a result list including the secondentity and the copy of the second entity in a ranked order; andreturning the result list in response to the request for access to thesecond entity that designated the second overlay dataset.
 4. Thecomputer-implemented method of claim 1, further comprising: modifyingthe first entity retrieved from the underlying base dataset, themodifying comprising copying the first entity into the first overlaydataset.
 5. The computer-implemented method of claim 4, furthercomprising: subsequent to modifying the first entity, receiving arequest for another access to the first entity, the request designatingthe second overlay dataset; determining, responsive to the other accessdesignating the second overlay dataset, that the first entity is notincluded in the second overlay dataset; determining that the firstentity is included in the first overlay dataset; and retrieving thefirst entity from the first overlay dataset based on determining thatthe first entity is included in the first overlay dataset, theretrieving comprising reading through the second overlay dataset to thefirst overlay dataset.
 6. The computer-implemented method of claim 4,wherein modifying the first entity comprises modifying at least one of avalue associated with the first entity, an attribute associated with thefirst entity, and a relationship associated with the first entity withrespect to the underlying base dataset.
 7. The computer-implementedmethod of claim 1, further comprising: modifying the first entityincluded in the underlying base dataset; and wherein retrieving thefirst entity from the underlying base dataset comprises reading throughthe second overlay dataset and the first overlay dataset to theunderlying base dataset to retrieve the modified first entity.
 8. Anon-transitory machine-readable medium having instructions storedthereon that, when executed by a processor, cause a computing device to:create a first overlay dataset that references an underlying basedataset included in a plurality of base datasets, the base datasetincluding a plurality of entities, the creating of the first overlaydataset including setting a dataset type attribute included in a firstdataset object associated with the first overlay dataset to an overlaydataset value, the overlay dataset value identifying a dataset as anoverlay dataset; create a second overlay dataset that references thefirst overlay dataset, the first overlay dataset, the second overlaydataset, and the plurality of base datasets being collections of dataincluded in a database, the creating of the second overlay datasetincluding setting a dataset type attribute included in a second datasetobject associated with the second overlay dataset to an overlay datasetvalue, the overlay dataset value identifying a dataset as an overlaydataset; receive a request for access to a first entity included in theunderlying base dataset, the request designating a first dataset;accessing the designated first dataset to determine whether the firstentity is included in the designated first dataset; responsive todetermining that the first entity is not included in the designatedfirst dataset, determine that the designated first dataset is the secondoverlay dataset based in part on determining that the dataset typeattribute included in the second dataset object is set to the overlaydataset value; access the first overlay dataset to determine whether thefirst entity is included in the first overlay dataset; responsive to theaccessing of the first overlay dataset: determine that the first entityis not included in the first overlay dataset; and determine that thedataset type attribute included in the first dataset object is set tothe overlay dataset value; retrieve the first entity from the underlyingbase dataset, the retrieving comprising reading through the secondoverlay dataset and the first overlay dataset to the underlying basedataset; and return the first entity retrieved from the underlying basedataset in response to the request for access to the first entity thatdesignated the first dataset.
 9. The non-transitory machine-readablemedium of claim 8, wherein the instructions, when executed by theprocessor, further cause the computing device to: receive a request foraccess to a second entity, the request designating the second overlaydataset; determine that the second entity is included in the secondoverlay dataset; retrieve the second entity from the second overlaydataset; and return the second entity retrieved from the second overlaydataset in response to the request for access to the second entity thatdesignated the second overlay dataset.
 10. The non-transitorymachine-readable medium of claim 9, wherein the instructions, whenexecuted by the processor, further cause the computing device to:determine that a copy of the second entity is included in the firstoverlay dataset; retrieve the copy of the second entity from the firstoverlay dataset; generate a result list including the second entity andthe copy of the second entity in a ranked order; and return the resultlist in response to the request for access to the second entity thatdesignated the second overlay dataset.
 11. The non-transitorymachine-readable medium of claim 8, wherein the instructions, whenexecuted by the processor, further cause the computing device to: modifythe first entity retrieved from the underlying base dataset, themodifying comprising copying the first entity into the first overlaydataset.
 12. The non-transitory machine-readable medium of claim 11,wherein the instructions that, when executed by the processor, cause thecomputing device to modify the first entity further include instructionsthat cause the computing device to modify at least one of a valueassociated with the first entity, an attribute associated with the firstentity, and a relationship associated with the first entity with respectto the underlying base dataset.
 13. The non-transitory machine-readablemedium of claim 8, wherein the instructions, when executed by theprocessor, further cause the computing device to: modify the firstentity included in the underlying base dataset; and wherein theinstructions that, when executed by the processor, cause the computingdevice to retrieve the first entity from the underlying base datasetfurther include instructions that cause the computing device to readthrough the second overlay dataset and the first overlay dataset to theunderlying base dataset to retrieve the modified first entity.
 14. Asystem comprising: at least one memory including instructions on acomputing device; and at least one processor on the computing device,wherein the processor is operably coupled to the at least one memory andis arranged and configured to execute the instructions that, whenexecuted, cause the processor to implement: creating a first overlaydataset that references an underlying base dataset included in aplurality of base datasets, the base dataset including a plurality ofentities, the creating of the first overlay dataset including setting adataset type attribute included in a first dataset object associatedwith the first overlay dataset to an overlay dataset value, the overlaydataset value identifying a dataset as an overlay dataset; creating asecond overlay dataset that references the first overlay dataset, thefirst overlay dataset, the second overlay dataset, and the plurality ofbase datasets being collections of data included in a database, thecreating of the second overlay dataset including setting a dataset typeattribute included in a second dataset object associated with the secondoverlay dataset to an overlay dataset value, the overlay dataset valueidentifying a dataset as an overlay dataset; receiving a request foraccess to a first entity included in the underlying base dataset, therequest designating a first dataset; accessing the designated firstdataset to determine whether the first entity is included in thedesignated first dataset; responsive to determining that the firstentity is not included in the designated first dataset, determining thatthe designated first dataset is the second overlay dataset based in parton determining that the dataset type attribute included in the seconddataset object is set to the overlay dataset value; accessing the firstoverlay dataset to determine whether the first entity is included in thefirst overlay dataset; responsive to the accessing of the first overlaydataset: determining that the first entity is not included in the firstoverlay dataset; and determining that the dataset type attributeincluded in the first dataset object is set to the overlay datasetvalue; retrieving the first entity from the underlying base dataset, theretrieving comprising reading through the second overlay dataset and thefirst overlay dataset to the underlying base dataset; and returning thefirst entity retrieved from the underlying base dataset in response tothe request for access to the first entity that designated the firstdataset.
 15. The system of claim 14, wherein the processor is furtherconfigured to execute the instructions that, when executed, cause theprocessor to implement: receiving a request for access to a secondentity, the request designating the second overlay dataset; determiningthat the second entity is included in the second overlay dataset;retrieving the second entity from the second overlay dataset; andreturning the second entity retrieved from the second overlay dataset inresponse to the request for access to the second entity that designatedthe second overlay dataset.
 16. The system of claim 15, wherein theprocessor is further configured to execute the instructions that, whenexecuted, cause the processor to implement: determining that a copy ofthe second entity is included in the first overlay dataset; retrievingthe copy of the second entity from the first overlay dataset; generatinga result list including the second entity and the copy of the secondentity in a ranked order; and returning the result list in response tothe request for access to the second entity that designated the secondoverlay dataset.
 17. The system of claim 14, wherein the processor isfurther configured to execute the instructions that, when executed,cause the processor to implement: modifying the first entity retrievedfrom the underlying base dataset, the modifying comprising copying thefirst entity into the first overlay dataset.
 18. The system of claim 17,wherein the processor is further configured to execute the instructionsthat, when executed, cause the processor to implement: subsequent tomodifying the first entity, receiving a request for another access tothe first entity, the request designating the second overlay dataset;determining, responsive to the other access designating the secondoverlay dataset, that the first entity is not included in the secondoverlay dataset; determining that the first entity is included in thefirst overlay dataset; and retrieving the first entity from the firstoverlay dataset based on determining that the first entity is includedin the first overlay dataset, the retrieving comprising reading throughthe second overlay dataset to the first overlay dataset.
 19. The systemof claim 17, wherein modifying the first entity comprises modifying atleast one of a value associated with the first entity, an attributeassociated with the first entity, and a relationship associated with thefirst entity with respect to the underlying base dataset.
 20. The systemof claim 14, wherein the processor is further configured to execute theinstructions that, when executed, cause the processor to implement:modifying the first entity included in the underlying base dataset; andwherein retrieving the first entity from the underlying base datasetcomprises reading through the second overlay dataset and the firstoverlay dataset to the underlying base dataset to retrieve the modifiedfirst entity.